1. Field of the Invention
The present invention relates generally to chemical detection, and in particular, to a method, apparatus, and article of manufacture for detecting chemicals using photonic crystal laser sources.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Various prior art techniques have been used to detect and determine the presence of a chemical in an analyte. An example of one such technique is the use of a semiconductor laser that is pumped such that the laser emission is projected into the analyte. The amount of refraction, emission, or spectra that is observed may then be used to determine the chemical composition within the analyte. However, such measurements may be inaccurate, require substantial amounts of analyte to perform the analysis, and may not provide sufficient sensitivity for detecting a particular substance.
Another example of a measurement technique is spectroscopy. Spectroscopy (such as infrared absorption spectroscopy [IR], or Raman spectroscopy) is a method often used to detect and identify substances (e.g., gases, liquids, or solids) such as toxic or explosive materials. To identify an unknown substance, the spectra (e.g., the wavelength and intensity) of light (that has been absorbed, emitted, or scattered) from the molecules of the unknown substance are measured. In this regard, the spectra of light provide a “fingerprint” that can be used to identify the molecules.
Spectroscopy utilizes the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules (or atomic or molecular icons) to qualitatively or quantitatively study the atoms or molecules, or to study physical processes. To measure spectral reflectance, a variety of different types of spectrometers may be used. In this regard, spectrometers often record a spectrum on a detector at a focal plane after a light ray/beam proceeds through a series of lenses, apertures, stops, and diffraction gratings.
The construction of compact spectroscopic tools for the optical analysis of ultra-small (˜10−18 liter) sample volumes remains an important goal in the development of integrated microfluidics systems. Miniaturization of appropriate light sources and detectors can enable very compact and versatile “laboratory on a chip” devices, in which many analytical functions can be monolithically combined. One of the device integration platforms which is ideally suited to enable such integration of ultra-small and efficient optical components is the membrane based planar photonic crystal, defined in high refractive index contrast materials by standard lithography and semiconductor fabrication processes. A photonic crystal is a fabricated material with a spatially periodic dielectric constant, for example, a dielectric slab with a lattice of holes etched in it. Some lattice types can exhibit a photonic bandgap, a range of wavelengths of light for which propagation through the material in certain directions is not allowed. A defect in the lattice, for example, a missing hole, can give rise to localized modes with wavelengths within the photonic bandgap, thus acting as an optical cavity.
High quality optical cavities with mode volumes far below a cubic wavelength may be used to obtain very high optical field intensities from ultra-small laser sources. Until recently, the applications of planar photonic crystals have been restricted to large-scale integration of optical wavelength division multiplexing (WDM) components for telecommunications. Compact lasers, detectors, modulators, waveguides and prisms have been fabricated and demonstrated in semiconductor slabs of silicon, GaAs or InGaAsP [1]. These devices have been used to generate, concentrate and route light efficiently within nanophotonic chips. Discrete planar photonic crystal nanocavities with high quality factors and small mode volumes have also been applied to cavity QED experiments [2]. These take advantage of the strong overlap between a spectrally narrow light emitter placed into the intense electromagnetic fields of a high finesse optical nanocavity.
However, to date, photonic crystal cavities have not been used to provide sensitive detection of chemicals. Further, prior art spectroscopy has many limitations including requiring the use of large amounts of analyte and a lack of sensitivity in determining the presence of certain chemicals.